The invention relates in general to a detector module for an X-ray detector system for use in X-ray holography and X-ray spectroscopy with atomic resolution as well as a modular X-ray detector system for the above applications in which such detector modules are used.
Since the invention of holography in 1948, work has been carried out on the application of the holography principle to the three-dimensional representation of atomic structures. A possible solution principle is based on the technique of X-ray holography.
In X-ray holography, atoms in a material sample to be examined are excited to fluorescence, and the fluorescence radiation from the material sample is recorded by a detector. The electric output signals of the detector, which reflect the interference field which builds up within the material sample, then give information about the three-dimensional structure of the examined sample material. For this purpose it is however necessary that the highest possible number of measurements be carried out on the material sample.
In the past few years, clear progress has been made in the development and production of X-ray detectors which are intended to record the fluorescence radiation of the material sample and separate this reliably from the varied background radiation. These detectors must on the one hand be energy-sensitive, in order to make possible a distinction of the incoming photons according to their energy or the wavelength of the radiation, but on the other hand, make it possible to operate up to such high counting rates that they record some hundreds of thousands of photons per second. For this purpose, in addition to silicon detectors, mostly germanium detectors were used in the past. The latter must however be cooled with liquid nitrogen, which is relatively costly, and are more suited to recording radiation from approximately 10 keV. A further disadvantage in the use of germanium detectors is that the electronics needed to amplify the measurement signals from the germanium detector can only be arranged at a position which is relatively remote from the germanium detector. To couple the germanium detector with the amplification electronics, long connection lines are thus required, which leads to strong interference and to a susceptibility to error. An integration of pre-amplifier stages in the vicinity of the germanium detector has to date not been successful, the costly cooling of the germanium detector representing a major obstacle. In addition, with an integration of the amplification electronics in the vicinity of the germanium detectors, a considerable number of signal lines must be routed away from the germanium detector or from the amplification electronics, which has proved to be an insurmountable obstacle even with smaller detector lines or detector arrays.
Very recently, progress has also been made in the development and production of location- and energy-resolving silicon X-ray detectors. Thus for example, the monolithic integration of highly sensitive drift detector cells with field effect transistors based on high-resistance silicon substrates was achieved. This detector type has already been used as a single-cell detector in the field of X-ray holography.
As mentioned above, it is necessary for X-ray holography that as large as possible a number of measurements of the material sample be carried out. In one of the possible concrete measurement processes (measurement process 1), this means that a large number of measurements of the fluorescence radiation of the sample are carried out above the material sample over the solid angle region of a hemisphere above the material sample with an angular resolution in the degree range. With these measurements, it is necessary when using single-cell detectors to displace the detector, by means of a mechanically complex and costly displacement structure, stepwise along various tracks on the semi-spherical surface above the material sample. To be able to detect the characteristic lines within the spectrum with the required accuracy, approximately 2xc2x7106 entries per solid angle element are for example required. Up to an event rate of approximately 150 kHz, the lines can be determined without major adverse effect on their width. As, for example, 7200 recordings at different solid angles may be required for a complete hologram, a total measurement time of some 24 hours results.
In a second concrete measurement process (measurement process 2) of X-ray holography, the required angular resolution is achieved through different arrival angles of monochromatic X-ray light. An angular resolution of the fluorescence radiation of the sample and thus a displacement of the detector is not required. Due to the abovementioned event-rate limitation of single-cell detectors, the same total measurement time results.
It is possible to shorten the long total measurement time by using multi-cell detectors instead of a single-cell detector. Through simultaneous measurement of different angle regions (measurement process 1) or the event rate correspondingly multiplied in the case of multi-cell detectors, (measurement process 2), the total measurement time is reduced by approximately the factor of the number of detector elements. Due to the limited number of cells or elements, all commercial multi-cell germanium detectors neither make costly displacement structures superfluous in measurement process 1 nor make possible a measurement time reduction to less than approximately one hour (both measurement processes). This reduction in measurement time is regarded as insufficient, as both (synchrotron) radiation sources and the detectors are subjected to fluctuations during long-time operation. The material sample itself can also change during this long measurement, for which reason real-time recordings are ideally desired.
In addition to the detailed example of X-ray holography presented here, detectors for X-ray radiation are used in many other measurement methods, for example in X-ray absorption spectroscopy, X-ray diffraction, X-ray fluorescence analysis and many more fields. For reasons comparable with those mentioned above, commercial silicon and germanium detectors limit the measurements in many applications (for example in synchrotron radiation sources) due to the maximum possible event rate of the detectors or the achievable angle or location resolution.
In U.S. Pat. No. 5,041,729, a multi-cell radiation detector is disclosed in which a number of detector elements is arranged in the form of a line. The radiation detector contains a scintillator on the rear side of which 12 lamellar photodiodes are arranged in the form of a line alongside each other. A holder is attached to the photodiodes by means of an isolating adhesive so that all 12 photodiodes are covered and an end section of each photodiode is exposed for wiring. The holder consists of a ceramic insulator and is equipped on its rear side with signal lines for each element. The bond-connection surface of each photodiode is connected by a wire bond to the signal lines. The disadvantage of this design is that only a detector linexe2x80x94but not a detector arrayxe2x80x94can be produced as the type of wiring allows exclusively a linear arrangement of the detector elements. When using a detector line however, a very long measurement time is required. In addition, the detector line must be displaced stepwise by means of a mechanically complex and costly displacement structure to carry out a complete recording.
It is therefore the object of the invention to provide a detector system with the help of which the above-mentioned disadvantages of the state of the art are overcome. It is in particular the object of the present invention to provide a detector module with a two-dimensional arrangement of detector elements forming a detector array including the corresponding wiring technique, the simultaneous recording of X-ray light being possible via a location or angular resolution, so that for example in X-ray holography, the otherwise customary displacement structure is superfluous. A further object of the invention is to make possible a high total event rate of the detector system so that the measurement time is clearly reduced for the same quality of the measurement results, or the quality of the measurement results is increased for the same measurement time.
A detector module with the features of patent claim 1 serves to achieve these objects, and also a detector system constructed from such detector modules with the features of patent claim 29. Advantageous versions of the detector module and of the detector system are the subject of the associated dependent claims.
The basic idea of the present invention is to arrange several detector modules, each containing a number of detector elements, around the material sample to be examined, roughly in the form of a hemisphere. On the basis of such a detector arrangement, significantly shorter measurement times and even the generation of real-time images is possible.
This arrangement of detector elements leads, however, due to the high necessary density of the detector elements, to subsequent problems with regard to the contact between the detector elements and the cooling of the detector modules or of the associated signal-processing electronics. In the case of the detector module according to the invention, detector cells are used which are provided with integrated pre-amplifier electronics on the silicon substrate. As a result, although the length of the signal line tracks between the detector element and the pre-amplifier step is reduced, which leads to a marked reduction in interference, the number of signal lines which must be routed away from the detector element/pre-amplifier arrangement is simultaneously increased.
The arrangement of the detector modules is preferably based on the capped icosahedron structure of C60 fullerene (buckyball), in which (as with a football) an internal basic framework in the form of a hemisphere is formed by ten hexagonal detector modules, the five pentagonal holes of which between the hexagonal detector modules are filled by five further detector modules. Either pentagonal detector modules can be inserted into the pentagonal holes or, which is preferable on cost grounds, the pentagonal holes covered by identical hexagonal detector modules.
In a preferred design, the radius of the hemisphere is approx. 3.7 cm. With this design, approx. 900 individual detector elements are provided in total, as a result of which a solid-angle resolution of approx. 4xc2x0 is achieved. With the capped icosahedron structure, 15 detector modules are required in total, each detector module bearing approx. 60 detector elements. As detector type, silicon detectors are used with which, in contrast to the known germanium detectors, operation is possible at room temperature. Silicon detectors are superior to germanium detectors at low and average energy levels and high counting rates in the resolution. However, each silicon detector element has a power consumption (determined by the transistor integrated therein) of up to approx. 4 mW, so that a power density of up to approx. 80 mW/cm2 results therefrom. It is clear from this that, despite the use of silicon detectors measures to cool the detector elements or the detector modules are required (conventional air, water or Peltier cooling).
As explained above, each detector module preferably contains approximately 60 detector elements which form a detector array in areal, side-by-side arrangement, which essentially has the same hexagonal basic shape as the actual detector module. Due to the necessary low-parasitism coupling of the signal-processing electronics for the processing of the analog signals coming from the individual detector elements (highly sensitive drift detector cells which are monolithically integrated with field effect transistors), these signal-processing electronics must be integrated in the vicinity of the detector elements or of the detector array. Because of the lower self-heating of the individual detector elements and the clearly higher loss consumption of the components of the signal-processing electronics, a thermal decoupling between the signal-processing electronics and the detector array and also a cooling or a good heat dissipation of corresponding heat flows is necessary. A good heat dissipation is achieved by arranging for housing parts of the module body to consist of a material with good heat-conducting properties, preferably graphite. A thermal decoupling of both heat sources is achieved by arranging for heat flows of the two heat sources to take the shortest possible common tracks. The selection of suitable materials and cross-sections between heat sources and heat sink is based on the respective quantities of the heat flows.
The local integration or low parasitism coupling of the signal-processing electronics to the detector array preferably takes place with the help of a conductor track carrier which is arranged directly above the detector array. On this conductor track carrier there would theoretically have to be provided a conductor track for each connection of each detector element, each conductor track having a first end contact in the immediate vicinity of the respective detector connection, in order to be connected to same by a bonding wire, and a second end contact which is located at one end edge of the conductor track carrier, in order to be connected from there to the electronics.
Upon the electric contact between the individual silicon detector elements, in the development presented here with integrated field effect transistor, contact between a total of 6 connections would be necessary for each detector element, and connection with very short bonding wire lengths to the conductor track carrier arranged above the detector elements. In the case of a sensitive surface of approximately 5 mm2 for each detector element, a contact with a conventional bonding wire technique is not possible. In addition, the conductor tracks lying tightly alongside another would mutually influence one another, which would lead to a marked impairment of the measurement results. This problem is solved according to the invention in that some connections of the detector elements, preferably the sensitive signal conductor connections, are preferably each connected together with a constant-voltage-carrying connection by bonding wires to the conductor track carrier or to the respective first end contacts of the conductor tracks provided on the conductor track carrier. The remaining connections of the detector elements are connected with the help of simple chain bond connections to an external bus structure running in annular fashion around the external edges of the detector array, and via bonding wires through additional bores in the edge area of the conductor track carrier to conductor tracks provided on the conductor track carrier. The number of bond connections which must be guided through bores in the conductor track carrier, and also the number of conductor tracks is clearly reduced as a result. To reduce the undesired reciprocal coupling of the conductor tracks arranged side-by-side on the conductor track carrier, the conductor tracks which are connected to the above constant-voltage-carrying connections are each guided on the conductor track carrier between two signal conductor tracks running side-by-side.
The coupling between signal lines is determined essentially by the dielectric constant of the carrier material. Ceramic materials such as Al2O3 or else AlN have a dielectric constant higher by roughly a factor of 3 than for example polymers. Therefore, an intermediate layer with a clearly smaller dielectric constant is preferably embedded between the mechanically stable carrier material with a higher dielectric constant and the signal-carrying metallization plane. The thickness of this intermediate layer should correspond approximately to the width of a signal-carrying conductor track. As material for the intermediate layer, there can be used in particular benzocyclobutenes or also polyphenylquinoxalines with a relative dielectric constant of approx. 2.7, but also standard polyimides would be conceivable. A further optimization is achieved by providing the screening conductor tracks located in the metallization plane (detailled description follows) at the same point also in a second metallization plane between the rigid support and the dielectric intermediate layer. Compared with the simplest solution, in which the signal lines are developed directly on a ceramic support, the coupling capacity can be reduced by more than a factor of 30, precisely when there are small gaps between the signal lines (for example approx. 50 xcexcm for conductor track widths of approx. 15 xcexcm).
As mentioned at the outset, the basic idea of the present invention is to arrange several detector arrays or detector modules, each containing a number of detector elements, around the material sample to be examined, roughly in the form of a hemisphere. The hemisphere surface is preferably constructed by an arrangement of several areal detector modules each of which contains several detector elements. The individual detector modules are preferably shaped and arranged such that as complete a coverage of the hemisphere surface as possible is achieved. The use of as few different module forms as possible is preferred. A possible variant is to combine four hexagons and five squares into a tetrakaidecahedron. A further preferred variant is to use a capped icosahedron structure of C60 fullerene (buckyball) in which an approximately hemispherical surface is formed from ten hexagons, the five pentagonal free surfaces of which are either closed by suitably inserted pentagons or covered by identical hexagons. With both variants named by way of example, it is preferred to construct the hemispherical surface from identical detector modules, for which reason the quadrangular free surfaces of the tetrakaidecahedron or the pentagonal free surfaces of the icosahedron structure are each covered by the hexagonal base modules, as a result of which only one single module type (a hexagonal detector module) is required with both variants. To better match the hemispherical surface, it is naturally also possible to use curved detector modules.
In the case of a preferred design of the invention, the abovementioned icosahedron structure is used with which the connections between the individual detector elements and the downstream signal-processing electronics (analog amplifiers etc.) can be kept very short in order to reduce parasitic effects in this manner. The chip(s) which contain the signal-processing electronics are preferably arranged above the detector module and radiometrically screened from the detector array. The connections between the connections of the detector elements of a detector array and the connections of the chips of the signal-processing electronics are produced with the help of a flexible flat cable or a flexible connection film which is led out close to a side edge of the hexagonal detector module.
As already explained above, the preferred detector module has a hexagonal shape and contains approx. 61 connected detector elements. The external edge length of the preferred module is approximately 1.5 cm and the active surface is approximately 3 cm2. The proportion of active to passive surface is approximately 50%, this proportion being dependent on the bonding technique used. In the case of so-called flip-chip contact, a proportion of active to passive surface of approximately 90% can be achieved. In the case of flip-chip contact, problems can arise due to the different coefficients of thermal expansion of AlN (conductor track carrier) and Si (detector array). A possible solution is the use of silicon also for the conductor track carrier, when a polymer layer for example can then be used as dielectric.
In the case of a preferred sphere radius of the buckyball arrangement of approx. 3.7 cm, an average solid-angle resolution of approx. 4xc2x0 results. It is apparent that the solid-angle resolution can be increased, while maintaining the detector element surface density, by increasing the hemisphere radius. However, the total number of detector elements and consequently also the technical outlay required for the processing of the many data channels, also increases at the same time. The solid-angle resolution can also be increased, while maintaining the hemisphere radius, by a reduction in the active surface per detector element (and consequently by an increase in the detector element surface density), limits being set to this miniaturization by the bonding techniques used.